An erythroblast is a type of stem cell
in the bone marrow
that gives rise to red blood cells
The presence of lots of erythroblast cells in a newborn baby's bloodstream is one of the symptoms of erythroblastosis fetalis, a serious anemia that can occur in newborns that have Rh+ blood and which are born to mothers with Rh- blood. If the mother's blood has been exposed to the Rh+ blood of her unborn child, her body then stages an immune response against the mistaken invading cells and her white blood cells attack the fetus' blood.
Some or all of the information in this writeup was taken from the science dictionary at http://biotech.icmb.utexas.edu/; I oversaw the development of the dictionary (the website was mothballed in 1998) and believe I wrote the entry this writeup is partly or wholly based upon.